Lallana was initially out of contract at the end of June but signed a short-term extension last month to see out the Premier League title-winning campaign in full with the Reds.
"I think Ads needs to take a lot of credit," Henderson told the Liverpool Echo. "When he first came to this club in 2014, he's been a massive part of the direction of change in philosophy to help make this club a success.
"It was difficult when he came and Brendan (Rodgers) left, so it was a transition for him. Since Jurgen came in, he was the best player. He moved more into the middle and was outstanding for Liverpool and England.
"Anyone who has played with or against him will tell you how good a footballer he is and I am not just saying that because he is my mate. Anyone who watches him knows how good he is.
"He had a bad injury, unfortunately, which took him a bit of time to recover and get back from and that was a tough time, you know.
"I could see how much that hurt him playing so well and so consistent to having a bad injury and being out for a long period of time.
"That is so difficult as a footballer. I have had that experience as a footballer and it is not nice. That was tough to see him down and out for so long but he has been back fit for a long time now and unfortunately for us he is going to move on now.
"But whatever team gets him is a big treat because he is a massive player. He is a great lad and a great leader in the group and the dressing room.
"And even if you haven't seen him much on the pitch, he still has a massive influence in the dressing room, so we will definitely miss him."